In Greek mythology, Aegisthus was the offspring of an incestuous relationship between prince Thyestes (son of Pelops) and his daughter Pelopia. Thysetes sired Aegisthus off of Pelopia in order to fulfill a prophecy given at Delphi which would allow him to avenge himself on his brother and rival Atreus.
When Pelopia gave birth to Aegisthus she was ashamed of what she had been forced into and abandoned the infant in the woods. The baby was found by a sheppard and then adopted by Atreus.
Many years later, when the truth was uncovered, Aegisthus killed Atreus and put Thyestes on the throne of Mycenae.
Thyestes was later killed by Agamemnon, who took the throne. When Agamemnon lead the Greek expedition to Troy, Aegisthus started an affair with the king’s wife, Clytemnestra and assisted her in murdering Agamemnon upon his return.
Aegisthus then ruled Mycenae until being killed by Agamemnon’s son, Orestes.
Notes: This is a part of the Greek Mythology Challenge. Day 24 - Myth Involving Murder.